1. Field of the Invention
The present invention relates to a dual mode display apparatus operating in ambient and back-light modes, or reflective and transmissive modes. More particularly, the present invention relates to a dual mode liquid crystal display apparatus utilizing a plurality of polarizers, including a wire grid polarizer, to selectively manipulate light.
2. Prior Art
A typical liquid crystal display device comprises a layer of liquid crystal material sandwiched between front and back transparent plates. Transparent electrodes are located on the inner surfaces of the transparent plates and used to apply electrical signals which alter the light transmission properties of the liquid crystal layer. The transparent electrodes are typically patterned to define the pixel structure of the display device. The surfaces of the transparent electrodes are also treated to ensure a preferred alignment direction for the liquid crystal molecules adjacent to each surface.
The preferred and most commonly used form of liquid crystal display utilizes the well-known "twisted nematic" liquid crystal effect. The twisted nematic effect is preferred because it offers excellent contrast ratio, low driving voltages and a sharp response threshold compatible with current drive circuit technology, wide viewing angle, and good gray-scale rendition.
In a typical twisted nematic liquid crystal display with a backlight, the display sandwich also includes linear polarizers affixed to the outer surfaces of the front and back transparent plates. The liquid crystal layer is aligned at the surfaces such that the polarization vector of light transmitted through the layer is rotated 90 degrees in the absence of an applied electric field, but is not rotated in the presence of an electric field.
The display sandwich also includes linear polarizers affixed to the outer surfaces of the front and back transparent plates. The transmission axis of the first polarizer is generally aligned in parallel to the orientation of the liquid crystal molecules adjacent to the front transparent plate. The transmission axis of the second polarizer is either parallel to, or orthogonal to, the transmission axis of the front polarizer.
In the case where the transmission axis of the two polarizers are orthogonal, the light transmitted through one polarizer is reoriented, in the absence of an electric field, to pass through the opposing polarizer such that the panel is transparent and appears bright to the observer. For this reason, a twisted nematic liquid crystal device with orthogonal polarizers is commonly referred to as operating in the "normally white", mode. In the presence of an electric field, the light transmitted by one polarizer is not rotated and is thus blocked by the second polarizer. Thus, the panel is opaque and appears dark to the viewer. In this manner, the transparent electrodes can be used to apply electric fields to selected areas of the panel to create a visible image in the form of light and dark pixels.
Twisted nematic liquid crystal displays constructed with parallel polarizers operate in what is commonly termed the "normally dark" mode of operation. In the absence of an electric field, the light transmitted through one polarizer is reoriented to be blocked or absorbed by the opposing polarizer such that the panel is opaque and appears dark to the observer. In the presence of an electric field, the light transmitted by one polarizer is not rotated and is transmitted by the second polarizer. Thus, the panel is now transparent and appears bright to the viewer.
In many applications, the liquid crystal display device is illuminated by a light source located behind the rear side of the sandwich and viewed from the opposing side. In this case, the visible image is created by light that passes through the panel a single time. In other applications, however, such as portable communications equipment, low power consumption is critical and the display is illuminated primarily by ambient light. In this case, a reflective element is located behind the liquid crystal sandwich such that the ambient light passes through the sandwich, reflects from the reflective element, and passes again through the sandwich in the opposing direction towards the viewer. Thus the image seen by the viewer is formed by light which has passed through the liquid crystal device twice.
The problems with current ambient-illuminated twisted nematic liquid crystal devices relate to the fact that the light passes through the device twice. The most significant problem, normally referred to as "parallax", occurs because the reflector is located behind the rear transparent plate and the rear linear polarizer at a considerable distance from the liquid crystal layer. The ambient light entering the display is spatially modulated by the liquid crystal layer to form a pattern of light and dark areas where the light impinges upon the rear reflector. After reflection, the light passes through the liquid crystal device in the reverse direction and is again spatially modulated. However, since the display is normally illuminated and viewed at oblique angles with respect to the display surface, the images formed by the two passes through the liquid crystal sandwich generally do not entirely overlap, and a double image, or shadow image, is seen by the viewer under most conditions. Although the shadow image is currently accepted for low resolution displays such as those used in portable phones and calculators, this phenomenon does limit the resolution, or minimum pixel size, of ambient illuminated twisted nematic liquid crystal displays, and prevents the application of such displays in high-information density applications, such as lap-top computers.
An additional problem with current ambient illuminated twisted nematic liquid crystal displays is the additional loss of brightness that occurs due to absorption in the linear polarizers. Note that this would not be a problem with theoretical polarizers that transmit 100% of one polarization while absorbing 100% of the orthogonal polarization. However, current linear polarizers only transmit 85% or less of the preferred polarization. The additional absorption during the second pass through the liquid crystal sandwich results in a loss of at least 30% of the possible display brightness.
A third problem with ambient illuminated liquid crystal display devices is that their performance must be compromised to allow illumination by an internal light when the ambient light is insufficient. The typical method to allow for internal illumination is to use a partially transmissive reflector, or transflector, behind the liquid crystal panel so that some light can be introduced to the display by a light source in back of the panel. Of course, the brightness in the reflective mode is reduced by the fraction of ambient light that is transmitted through this transflector, and the amount of illumination required to adequately light the display from the back must be increased to offset the light reflected back to the source by the transflector.
Alternate methods have been proposed to eliminate the parallax problem in ambient illuminated twisted nematic liquid crystal displays. One method, as described in U.S. Pat. Nos. 4,492,432 and 5,139,340, is to utilize an alternate liquid crystal electro-optical effect that only requires a polarizer on the front side of the display. Since the rear polarizer is not required, the rear reflector can be located on the inner surface of the rear transparent plate in immediate proximity to the liquid crystal layer. While this method eliminates the parallax problem, displays using this method do not provide the high contrast, wide viewing angle, fast response, and smooth gray scale rendition provided by twisted nematic liquid crystal display devices. In addition, these displays cannot be illuminated from the back when the ambient light is insufficient.
Still another method is the Polymer Dispersed Liquid Crystal Display (PDLC) in which the liquid crystal layer itself functions as a diffuse reflector, eliminating the need for polarizers or a separate reflector. While this method offers the potential for high display brightness, the PDLC requires high drive voltages and complex drive waveforms that are not compatible with current drive circuit technology. There is still no alternative which provides the high image contrast, wide viewing angle, fast response, and smooth gray scale rendition provided by twisted nematic liquid crystal displays. Clearly, it would be desirable to develop a method for dealing with parallax in the display while preserving the advantages of the twisted nematic liquid crystal display.
One attempt in the art to overcome some of the foregoing disadvantages is provided in U.S. Pat. No. 4,688,897, to Grinberg, which attempts to improve ambient illuminated twisted nematic liquid crystal displays by incorporating a wire grid reflective polarizer within the twisted nematic liquid crystal device. The wire grid reflects light polarized along the long axis of the wires, and transmits light of the orthogonal polarization. The transmitted light must be absorbed within or behind the display device to provide a high contrast display. The wire grid functions as the rear polarizer, as a specular reflector, and as the rear electrical contact to the liquid crystal layer.
Despite any advantages obtained, the above device was primarily described for operation with ambient light. Although a backlight was provided, the above device does not offer a solution as to how to absorb the ambient light transmitted through the wire grid reflective polarizer while, at the same time, not absorbing or attenuating the light from the source behind the panel. Note that backlights for liquid crystal displays are typically complex optical systems designed to capture the light from one or more lamps and distribute the light uniformly over the surface of the display. These backlights commonly include components such as light guides, diffusers, and reflectors. Thus, unless the ambient light transmitted through the wire grid polarizer is substantially absorbed, some portion of this light will reflect from the backlight components and pass back through the display device, thus lowering the display contrast. Therefore, a display device as described by Grinberg would require a compromise between contrast ratio in the ambient-lit mode of operation and brightness in the back-lit mode.
Other reflective polarizer technologies have been developed, including cholesteric polarizers and multilayer birefringent polymer films. In the future, these types of reflective polarizers may be used within liquid crystal devices in a manner that eliminates the parallax problem, but there still exists a need for an improved ambient-illuminated display that overcomes the parallax problem while maintaining the performance advantages of the twisted nematic liquid crystal effect. In addition, there still exists a need for a dual mode ambient-illuminated and internally-illuminated display.
Therefore, it would be advantageous to develop a liquid crystal display apparatus with reduced parallax. It would also be advantageous to develop a liquid crystal display apparatus capable of high brightness operation with either ambient illumination or internal illumination, without compromising the display contrast in either mode of operation.